The Hamilton-Norwood scale is a way of measuring male pattern baldness. It was introduced by Dr. James Hamilton in the 1950s and later revised and updated by Dr. O'Tar Norwood in the 1970s.
There are 2 main types of balding, anterior (front) and vertex (back), as this alternate view of the Norwood scale shows a bit more clearly.
Stages on the scale are often abbreviated to nw1, nw2, nw3a etc. Another slight variation on the scale is shown below.
Baldness in women takes a different pattern, and is measured on the Ludwig Scale.
I intend to give each of my postings a rating on the Norwood scale, but remember that this is only personal opinion and sometimes I may get it wrong. The Norwood scale is a less than perfect way of measuring baldness, but it is the best and most widely recognised way we have.
An alternative look at the Norwood Scale can be found at shortandbald.net